Conventional gas turbine engines burn natural gas, with the hot gasses from the combustion of the natural gas driving a turbine. The heated gases (typically about 1000 degrees Fahrenheit) exiting the turbine pass through an exhaust duct to a heat recovery steam generator (HRSG). The heat from the gasses is then recovered in the HRSG to produce steam, which in turn is used to drive a steam turbine. The cooled gases exit the HRSG to be exhausted through a short stack.
Conventional gas turbines are equipped with a set of dampers or diverters that allow the turbine to operate in either a simple-cycle or a combined-cycle mode. The bypass damper controls flow through the bypass or simple-cycle stack, and the isolation damper controls flow through the HRSG. During start-up operations, the isolation damper is closed, thereby preventing flue gas flow through the HRSG, and the bypass damper is open, allowing flue gas to exit through the bypass stack. This is referred to as simple-cycle operation. Once the turbine has completed start-up procedures the isolation damper is opened and the bypass damper is closed, consequently redirecting flue gas flow through the HRSG. The hot flue gas heats boiler feed water to produce steam, which, once it has reached sufficient quality, is used to drive a steam turbine to produce more electricity. This is referred to as combined-cycle operation.
Gas flow diverters are often employed to deliver hot exhaust gases from a turbine either to a HRSG or to the stack. Many problems are attendant their use due to the large volumes of gases at high temperatures and under substantial pressure that are exhausted by turbines and the necessity that the hinged gas diverter blade be swung into and out of its operative positions relative to the stack and to the steam generator without vibrating and without causing thermal shock on the system. Such diverters are large as are the conduits leading therefrom to the steam generator or to the stack. Consequently, as the conduit to the steam generator must be capable of being safely entered by service personnel, leakage into the HRSG must be prevented when gas flows are diverted to the stack in order to avoid the necessity of placing the turbine out of service.
The diverter dampers that are currently used in gas turbine systems operate to divert the hot exhaust from the gas turbine into the HRSG when the power plant is operated as a combined-cycle facility or into the exhaust stack in the case of simple-cycle operation. Such diverter dampers are typically quite expensive and do not provide long term reliability. Because any maintenance or modification work that may be performed within the HRSG requires that the HRSG be “human-safe,” any leakage from a diverter damper cannot be tolerated. As a result, some manufacturers include an isolation guillotine damper as the back the diverter damper. In using a guillotine damper, the guillotine blade is exposed to very high temperatures on one side and much cooler air on the opposing side, which can lead to warping of the blade. When warping of the guillotine blade occurs, leaks may be created in the diverter damper that allow hot and toxic gasses pass through to the HRSG and consequently endanger the health of the worker in the HRSG.
What is desired, then, and not found in the prior art, is a gas turbine by-pass system that effectively and securely diverts extremely hot gasses from the HRSG in simple mode to provide workers with the safe environment needed to perform their function within the HRSG, and that can further be operated in an inexpensive mariner.